An article made of an alloy having a shape memory can be deformed at a low temperature from its original configuration. Upon application of heat, the article reverts back to its original configuration. Thus, the article "remembers" its original shape.
For example, in nickel-titanium alloys possessing shape memory characteristics, the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change in temperature. This transformation is often referred to as a thermal elastic martensitic transformation. The reversible transformation of the Ni-Ti alloy between the austenite to the martensite phases occurs over two different temperature ranges which are characteristic of the specific alloy. As the alloy cools, it reaches a temperature (M.sub.s) at which the martensite phase starts to form and finishes the transformation at a still lower temperature (M.sub.f). Upon reheating, it reaches a temperature (A.sub.s) at which austenite begins to reform and then a temperature (A.sub.f) at which the change back to austenite is complete. In the martensitic state, the alloy can be easily deformed. When sufficient heat is applied to the deformed alloy, it reverts back to the austenitic state, and returns to its original configuration.
Titanium and nickel-titanium base alloys capable of possessing shape memory are widely known. See, for example, Buehler U.S. Pat. No. 3,174,851 issued Mar. 23, 1965, and Donkersloot et al., U.S. Pat. No. 3,832,243, issued Aug. 27, 1974. Commercially viable alloys based on nickel and titanium having shape memory properties have been demonstrated to be useful in a wide variety of applications in mechanical devices.
Albrecht, et al., U.S. Pat. No. 4,412,872 issued Nov. 1, 1983 indicates that memory alloys based on Ni-Ti possess an M.sub.S temperature which cannot, for theoretical reasons, exceed 80.degree. C., and in practical cases usually does not exceed 50.degree. C. Conventional nickel-titanium alloys are therefore unsuitable for use in high temperature applications such as heating, ventilating and air conditioning applications, which require M.sub.s temperatures exceeding about 80.degree. C. (176.degree. F.).
Nickel-titanium base alloys have been modified to obtain different properties. For example, it is known that higher transitions can be obtained by substituting gold, platinum, and/or palladium for nickel. See, Lindquist, "Structure and Transformation Behavior of Martensitic Ti-(Ni,Pd) and Ti-(Ni,Pt) Alloys", Thesis, University of Illinois, 1978 and Wu, Interstitial Ordering and Martensitic Transformation of Titanium-Nickel-Gold Alloys, University of Illinois at Urbana-Champaign, 1986. Additions of these elements, however, make the ternary alloys quite expensive. Tuominen et al., U.S. Pat. No. 4,865,663 issued Sep. 12, 1989, discloses high temperature shape memory alloys containing nickel, titanium, palladium and boron. Nenno, et al., U.S. Pat. No. 4,759,906 issued Jul. 26, 1988 discloses a high temperature shape memory alloy comprising 40-60 atomic % Ti, 0.001-18 atomic % Cr, and the balance being Pd. Donkersloot et al. U.S. Pat. No. 3,832,243, issued Aug. 27, 1974, describes a variety of Ni-Ti shape memory alloys, including Ni.sub.5 Ti.sub.4 Zr.
Various other additions to the conventional nickel-titanium alloy are known. For example, iron, copper, niobium and vanadium have each been suggested additives for various reasons. See, Harrison, U.S. Pat. No. 4,565,589 issued Jan. 21, 1986 which discloses a low M.sub.S alloy having from 36-44.75 atomic % nickel, from 44.5-50 atomic % titanium and the remainder copper; Harrison, U.S. Pat. No. 4,337,090 issued Jun. 29, 1982; and Quin, U.S. Pat. No. 4,505,767 issued Mar. 19, 1985. Melton, et al., U.S. Pat. No. 4,144,057 discloses a shape memory alloy consisting essentially of a mixture of 23-55 wt. % nickel, from 40-46.5 wt. % titanium and 0.5-30 wt. % copper, with the balance being from 0.1 to 5 wt. % of aluminum, zirconium, cobalt, chromium and iron.
Two Russian articles discuss the effect of various elements on the conventional nickel-titanium base alloy. "Calculation of Influence of Alloying on the Characteristics of the Martensitic Transformation in Ti-Ni", (D.B. Chernov, 1982) discloses the results of studies wherein the interaction of some 32 elements with nickel and titanium were calculated using experimental phase diagrams and on the basis of empirical methods. These calculations were then compared with known experimental data for ternary alloys of Ni-Ti and Nb, Cr, Fe, Co, Pd, Cu, Al, Si and Ge. The author concluded that one may expect that the martensitic peak temperature (M.sub.p), per one atomic % of the alloying component, may be raised or lowered by the addition of Cr, Ag, Au, Hf and Sc.
Another Russian article entitled "Martensitic Transformation in Alloyed Nickel-Titanium" (1986) identifies the results of x-ray diffraction studies of structural transformations in nickel-titanium alloys alloyed with transition elements. The article discloses that when titanium is replaced by zirconium and hafnium, the martensitic transformation in Ni-Ti is conserved, but with significant lowering of the M.sub.S temperature. The composition of the disclosed alloy is Ni.sub.50.5 Ti.sub.46 Hf.sub.3.5.
Many methods of forming shape memory alloys are known. For example, Thoma, et al., U.S. Pat. No. 4,881,981 issued Nov. 21, 1989, relates to a method of producing shape memory alloys. The method includes the steps of increasing the internal stress level, forming the member to a desired configuration, and heat treating the member at a selected memory imparting temperature. Other processing methods are taught by Wang, et al., U.S. Pat. No. 4,304,613 issued Dec. 8, 1981, and Fountain, et al., U.S. Pat. No. 4,310,354 issued Jan. 12, 1982.
Donachie, et al., U.S. Pat. No. 4,808,225 issued Feb. 28, 1989, discloses a process similar to that of Fountain, et al., but which comprises the steps of providing metal powder having at least 5 wt. % of one or more reactive elements such as titanium, aluminum, hafnium, niobium, tantalum, vanadium and zirconium. The powder is consolidated to an essentially fully dense shape, and then, localized areas of the consolidated shape are progressively melted and solidified to produce a product of improved ductility. Nickel-titanium alloys containing at least 45 wt. % nickel and at least 30 wt. % titanium are preferred. None of these known processing methods provide Ni-Ti alloys usable in high temperature applications.
The present invention addresses the problems and disadvantages of the prior art and provides a high transformation temperature shape memory alloy which has good strength characteristics and is more economical to use than the commercially available high temperature SMA.